This invention relates to optical systems and more particularly, the invention relates to the suppression of higher-order transverse modes in a multi-mode resonant cavity of a ring laser gyroscope.
One of the most significant ring laser gyroscopes yet proposed and constructed employs four waves of two pairs each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657, 3,854,819 and 4,006,989 to Keimpe Andringa and assigned to the present assignee. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beams, propagating in the clockwise direction includes both left-hand circularly polarized (LCP) waves and right-hand circularly polarized (RCP) waves as do those waves propagating in the counter-clockwise direction. The four-frequency or multi-oscillator ring laser gyro provides a means of circumventing the frequency locking or lock-in problem present in all conventional or two-frequency laser gyroscopes. This lock-in phenomenon occurs when two traveling waves propagating in opposite directions in a resonant cavity at slightly different frequencies are pulled toward each other to combine in a single frequency standing wave. However, when the frequencies of the counter-rotating waves are sufficiently separated in frequency, the pulling together does not occur. The four-frequency approach may be described as two independent laser gyros operating in a single stable resonator cavity, sharing a common optical path, but statically biased in opposite senses by the same passive bias element. In the differential output of these two gyros, the bias then cancels, while any rotation-generated signals add, thereby avoiding the usual problems due to drifts in the bias and giving a sensitivity twice that of a single two-frequency gyro. Because the bias need not be dithered, the gyro never passes through lock-in. Hence, there are no dither-induced errors to limit instrument performance. For this reason, the four frequency gyro is intrinsically a low noise instrument, and it is well suited for applications requiring rapid position update or high resolution.
The four different frequencies are normally generated by using two different otical effects. First, a crystal polarization rotator may be used to provide a direction-independent polarization causing the resonant waves to be circularly polarized in two directions. The polarization rotation results from the refractive index of the rotation medium being slightly different for RCP and LCP waves. Alternatively, a non-planar ring path may be used which inherently supports only circularly polarized waves without the use of a crystal rotator. A non-planar electromagnetic wave ring resonator is shown and described in U.S. Pat. No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner and assigned to the present assignee. Second, a Faraday rotator is used to provide non-reciprocal polarization rotation, by having a slightly different refractive index for clockwise (cw) traveling waves than for counter-clockwise (ccw) traveling waves. This causes the cw and ccw RCP waves to oscillate at slightly different frequencies while the cw and ccw LCP waves are similarly but oppositely split. Thus, a laser gyro operates with right circular polarized waves biased in one direction of rotation and with left circular polarized waves biased in the opposite direction, the bias being cancelled by subtracting the two outputs.
In the resonant cavity of a ring laser gyroscope, there are a number of resonant modes many of which are unwanted and must be suppressed. In the prior art, the suppresson of unwanted modes has been accomplished by machining a narrow spatial aperture into the gyro block cavity, preferably opposite a spherical mirror in a three mirror cavity. Another approach has been to insert a copper disk with a center hole into a resonant cavity as part of a Faraday rotator assembly. Still another approach, in the prior art has been to rely on the resonant cavity wall imperfections inherently present from the machining process of a laser gyro block. A major drawback of the prior art mode discrimination or suppression approaches has been that the aperture was not adjustable, thereby preventing fine tuning once the gyro block was machined and assembled; in addition, a scattering of the intercepted light waves occurred causing an increase in lock-band occurrences at high annular rotation rates and a variation in the gyro bias. These variations degrade the performance of a ring laser gyro.